The field of this invention is membrane gas separation apparatus and processes for recovering a methane-enriched gas from a mixture comprising methane and carbon dioxide.
To date a number of membrane gas separation processes have found commercial acceptance because of the compatability of specific membrane gas separation operations in the economic feasibility of operating such processes. Such processes include the recovery of hydrogen from purge gas streams, for instance from ammonia production processes or from hydrogen treating processes. In some instances separation of carbon dioxide and methane by membrane permeation is also economically feasible depending on the value of gases recovered in those operations in particular industries. One source of gas mixtures of carbon dioxide and methane is sanitary landfill wells which recover gas mixtures which are generated by the decomposition of solid waste in sanitary landfills; other sources include plant and/or animal waste digesters, including sewerage treatment plants and the like.
Such gases, for instance from sanitary landfills, are usually available at a well head pressure of less than about 2 atmospheres. Separation of such gases to natural gas pipe line quality methane by a membrane gas separation process usually requires compression of the well head gas mixture from such low pressure for instance, about atmospheric pressure, to substantially high pressure, for instance greater than about 100 psia. Depending on the membrane utilized and its permeation characteristics, it may be desirable to compress such low pressure gas mixture up to as high as 600 psia or even higher. The costs of compression often are substantially high and in some instances may preclude the economic feasibility of a membrane gas separation to recover methane from gases generated by sanitary landfills, digesters and the like. What has been discovered is an effective process and apparatus for recovering methane as a non-permeating gas by membrane gas separators, where substantial quantities of a carbon dioxide-containing permeate gas can be utilized as combustion fuel to provide power for compression of feed gas mixtures to be separated.
In describing the present invention a particularly oonvenient analytical characteristic of polymeric gas permeable membranes includes the permeability of the membrane for a specific gas through the membrane. The permeability (P.sub.a /l ) of a membrane for gas "a" of a gas mixture through a membrane of thickness "l" is the volume of gas, referred to standard temperature and pressure (STP), which passes through the membrane per unit of surface area of membrane, per unit of time, per unit of differential partial pressure of the permeating species across the thickness of the membrane. One method for expressinq permeabilities is cubic centimeters (STP) per square centimeter of membrane area per second per differential partial pressure of 1 centimeter of mercury across the membrane thickness (cm.sup.3 (STP)/cm.sup.2 -sec-cmHg). Unless otherwise noted, all permeabilities are reported herein at standard temperatures and pressures of 60.degree. F. and 14.7 psia, respectively. Permeabilities are generally reported in gas permeation units (GPU), which are cm.sup.3 (STP) cm.sup.2 -sec-cmHg.times.10.sup.6 ; thus 1 GPU is 1.times.10.sup.-6 cm.sup.3 (STP) cm.sup.2 -sec cmHg. Another convenient relationship for expressing gas permeation characteristics of membranes is separation factor. A separation factor, .alpha. a/b, for a membrane for a given pair of gases "a" and "b" is defined as the ratio of the permeability (P.sub.a /l) of a membrane of thickness "l" for a gas "a" of a gas mixture to the permeability (P.sub.b /l) of the same membrane to gas "b".
In practice, separation factor with respect to a given pair of gases for a given membrane can be determined by employing numerous techniques which provide sufficient information for calculation of permeabilities for each of the gases. Several of the many techniques available for determining permeabilities and separation factors are disclosed by Hwang et.al., Techniques of Chemistry, Volume VII, Membranes in Separations John Wiley & Sons, 1974 (herein incorporated by reference), at Chapter XII, pages 296-322.
Measurements can be made for pure gas permeation or for blend gas permeation. However, experience has shown that the measured permeability of a membrane for a gas species is higher for pure gas permeation than for blend gas permeation. In general, it is more desirable to determine gas permeation characteristics for blend gases, since the permeabilities and separation factors more closely predict actual membrane gas separation performance characteristics.
When a plurality of membranes are assembled into a separator in modular form, it is generally convenient to establish some standard sizes for such modularized membrane gas separators in terms of the amount of membrane surface area. For instance, one standard separator could have 150 square meters of membrane surface area while another standard separator could have 10,000 square meters of membrane surface area. When dealing with modularized membrane gas separators a convenient way of characterizing the permeation performance of a standard membrane gas separator is in terms of modular permeability which is the product of membrane surface area (A) and permeability (P/l). Unless otherwise stated modular permeability is expressed in modular flow units (MFU) which are cm.sup.3 (STP)/sec-cmHg. For instance a separator having a membrane surface area of 1000 square meters (10.sup.7 cm.sup.2) where the membrane exhibits a permeability for gas "a" of 25.0 GPU (that is, 25.times.10.sup.-6 cm.sup.3 (STP) cm.sup.2 -sec-cmHg), exhibit a modular permeability for gas "a" of 250 MFU.